1. Technical Field
Embodiments of the invention relate generally to power converters. Other embodiments relate to controlling power converter thermal cycling.
2. Discussion of Art
Trains typically feature a number of cars that are pushed or pulled by a locomotive. The locomotive has traction wheels engaged with the track. In modern designs, electric wheel motors drive the traction wheels. The electric wheel motors are powered via electrical distribution from one or more engine-driven generators housed within the locomotive. The traction wheels and wheel motors can be reversibly configured, to also act as brakes for slowing the locomotive.
Similarly, in the mining industry, off-highway vehicles (“OHVs”) usually employ electrically motorized wheels for propelling or retarding the vehicle. In particular, OHVs typically include a large horsepower diesel engine (or other engine) in conjunction with an alternator, a main traction inverter, and a pair of wheel drive assemblies housed within the rear tires of the vehicle. The diesel engine is directly associated with the alternator such that the diesel engine drives the alternator. The alternator powers the main traction inverter, in which semiconductor power switches commutate the alternator output current to provide electrical power to electric drive motors of the two wheel drive assemblies.
In both locomotive and OHV applications, solid state power converters are used to provide high voltage current from the generators or alternators to the wheel motors. Such power converters include inductive coils to step down the voltage as well as semiconductor power switches to commutate the current. Although the above-described applications are typical, it will be appreciated that power converters can be used in many other settings.
Generally, operation of a power converter is accomplished by applying alternately two different gate voltage levels to individual semiconductor power switches via corresponding gate drive units. It is a known problem that semiconductor power switches are subject to cyclic thermal stresses. While driven by a gate voltage, each power switch conducts significant current in a forward direction at a relatively small voltage drop across the switch. Despite the relatively low voltage across the forward-biased power switch, resistive heating nonetheless occurs. When gate voltage is removed, each semiconductor ceases to conduct (except for leakage current). Thus, with proper thermal design, a power switch not driven by gate voltage should cool toward ambient temperature.
Although durability is a consideration in semiconductor device design, electrical design constraints entail that the various layers of the semiconductor power switches are fabricated from materials having differing thermal properties; in particular, differing coefficients of thermal expansion. Thus, over time, thermal stress can cause mechanical failure modes such as delamination, debonding of terminals, or fatigue cracking. Thermal stress also can cause electrochemical failure modes such as current filamenting and Kirkendall void formation. Thermal stress effects can be rendered more predictable, and can be mitigated, by maintaining the heating/cooling cycle within a design envelope defined to minimize temperature swings despite continual ON/OFF cycling.
Therefore, in order to mitigate thermal stresses in semiconductor power switches, it is desirable to regulate thermal cycling.